An ATR infrared spectrometer is known for analyzing the chemical composition of a sample. The ATR infrared spectrometer (ATR: attenuated total reflection) has an elongated ATR crystal as an optical waveguide, in which infrared light is guided under total internal reflection, wherein approximately ten multiple reflections occur. Evanescent waves which interact with the sample arranged in the vicinity of the interface, for example on a sample stage, are formed behind the reflecting interface of the ATR crystal at the points of total internal reflection. By way of example, the material used for the ATR crystal is zinc sulfide or zinc selenide.
Arranged at the one longitudinal end of the ATR crystal is an infrared light source, by which the infrared light is coupleable into the ATR crystal. Arranged at the other longitudinal end, which is arranged distant from the one longitudinal end, is an infrared light sensor with a linearly variable wavelength filter, by which the spectrum of the infrared light decoupled from the ATR crystal is detectable. The zinc sulfide or the zinc selenide for the ATR crystal is used for wavelengths of the infrared light between 5.5 μm and 11.0 μm, wherein the linearly variable wavelength filter is tuned to this wavelength range in order to provide the corresponding spectral resolution using the infrared light sensor. By way of example, the infrared light sensor is a linear array made of a plurality of pyroelectric infrared light pixels. In order to obtain good illumination of the ATR crystal, the infrared light source is known to have a longitudinal extent which is at least as large as the longitudinal extent of the linear infrared light sensor array. Alternatively, use of a collimating lens between the infrared light source and the ATR crystal to focus the incident infrared light such that the linear infrared light sensor array is well-illuminated is known. Although this achieves a high spectral resolution of the ATR infrared spectrometer, the signal-to-noise ratio disadvantageously varies greatly over the relevant wavelength range of 5.5 μm to 11.0 μm.
FIG. 4 is a diagram which shows a curve of the signal-to-noise ratio of the ATR infrared spectrometer along the infrared light sensor array. The ordinate, denoted by reference numeral 16, specifies the signal-to-noise ratio, which is plotted over the abscissa 15, on which the positions of the individual infrared light pixels of the infrared light sensor array are shown in enumerated fashion. Infrared light with a wavelength of 5.5 μm is incident on the infrared light pixel with the position number 1 and infrared light with a wavelength of 11.0 μm is incident on the infrared light pixel with the position number 130. Incident on the infrared light pixels with the position numbers between 1 and 130 is infrared light with a wavelength between 5.5 μm and 11.0 μm, with the wavelength increasing linearly from the infrared light pixel with the position number 1 to the infrared light pixel with the position number 130. As can be seen in FIG. 4, the signal-to-noise ratio of the first 20 pixels is advantageously high. The signal-to-noise ratio is likewise still relatively high for the infrared light pixels with position numbers between 100 and 120. The signal-to-noise ratio is particularly low for the infrared light pixels with position numbers between 25 and 60. This uneven distribution of the signal-to-noise ratio over the infrared light pixels and hence over the wavelength range measured by the ATR infrared spectrometer is very disadvantageous, particularly if a signal-to-noise ratio which is as unchanging as possible over the whole wavelength measurement range is required for high accuracy of the analysis of the chemical composition of the sample.